Project Summary Nature uses vitamin B12 and its derivatives to perform fascinating chemical reactions that are essential for life. Enzymes that harness the reactivity of vitamin B12 are known for having dynamic transitions that move the co- factor between multiple sites in the protein. One such enzyme is the complex formed by the corrinoid iron-sul- fur protein and its methyltransferase (CFeSP/MeTr) from a microbial carbon ?xation pathway that is responsi- ble for producing an astounding 1012 kg of acetate annually. Microbes that utilize this pathway can be found in many contexts, including the human gut. Furthermore, a B12-dependent methyl transfer reaction is performed by the homologous enzyme methionine synthase (MetH) in humans to produce methionine from homocysteine. Prior studies of CFeSp/MeTr using X-ray crystallography suggest that large domain motions are an essential component of the enzyme turnover mechanism. Remarkably, enzyme turnover has been shown to occur within the crystal, and the motion of the B12 domain is the largest known for any protein in the crystalline state. We propose to develop new experimental and theoretical methods for X-ray scattering from MetH in solution and diffuse scattering from CFeSP/MeTr crystals to understand how long-ranged correlated motions in B12-depen- dent metalloenzymes regulate catalytic activity.